The Last Three Days

When I incubate eggs almost all of my eggs are fertile and grow correctly for the first 18 days. But when it is time for them to hatch I have maybe (if I'm lucky) 50% of the eggs hatch . When they do start to hatch it takes them at least three days for them all to hatch out. Why do I have so many deaths the last 3 days and why do they hatch so far apart from one another? Any help or advice is greatly appreciated and thanks in advance.

Probably humidity if they are good until lockdown. Half of them hatching is success too though I've had times where all that are going to hatch come at one time, some hatch on hatch day and some on the next day, and every now and then I'll have just one that hatches alone two days after everyone else. It may seem crazy to us but the chicks know when they are ready. I have waited until everyone I thought was going to hatch came out and started eggtopsies on the eggs that showed no sign of life for days only to find that I killed one because it was behind in development than the others. If given another day or two it may have hatched- especially if I hadn't slit the membrane. It happens and sometimes we don't know the reason but 50% is better than none! (Disappointing yes but probably for the best.) Watch for fluctuations in humidity next time to see if that is it. Anyone else?

this little one went all the way but couldn't hatch. I candled it and it was all dark with no air cell. No signs of life- membrane was totally intact. The chick was just too huge, left no air cell and had no room to hatch. Eggtopsies may help you determine what is going on with them.

I do eggtopsies on any egg that doesn't hatch and it is always a fully developed chick (dead one of course). I have a Little Giant Styrofoam incubator and I fill all the water trays full. In my last hatch I even added a small cup with paper towels and had 12 out of 21 hatch.